Update 2007-10-19: as our readers Aaron and Kai have kindly pointed out to me, this is no longer an issue. Hyphens & underscores are now treated equally by Google.
Well, here’s something I wasn’t aware of until moments ago. When using SEO-friendly and user-friendly URLs, always go for hyphens instead of underscores.
Why? Because according to this post, Google doesn’t recognize the underscore as a divider. Basically, that means you would have to include the underscore in your search term to bring up the URL. I don’t think anybody actually does that. Clearly a point for Team Hyphen. I’m just glad WordPress does “good” URLs out of the box.